mostly because the algorithms are known and can be optimized well against the GPU architecture.

GPUs are really good at parallel operations. so when it comes to calculating a bunch for similar values (ie. nonces for mining) it essentially allows for the effect of multiple CPUs because it can pipeline everything well.

the difficulty in GPUs is that they require the data be loaded in and known well in advance to extract and parallelize.

The TLDR is that GPUs are optimized for certain mathematical operations that cryptomining also uses. There is an intersection between the kind of math needed for graphics and the kind of math needed for cryptomining.

The AMD Ryzen or Intel I5 or w.e you have is a 'general' CPU. It is good at lot of tasks, but necessarily always the best and most efficient at most tasks.